1. Field of the Invention
The present invention relates to a nitride semiconductor light emitting device, and more particularly, to a nitride semiconductor light emitting device having an electrode structure that facilitates uniform current spreading.
2. Description of the Related Art
In general, a nitride semiconductor is a group III-V semiconductor crystal such as GaN, InN and AlN, and especially used widely as a light emitting device capable of generating a single wavelength light (ultraviolet rays or green light), and particularly, blue light. Such a nitride light emitting device is manufactured using an insulation substrate such as a sapphire substrate or a SiC substrate that satisfies lattice matching conditions for crystal growth. Thus, the nitride light emitting device typically has a planar structure in which two electrodes connected respectively to p- and n-nitride semiconductor layers are disposed substantially horizontally on an upper surface of a light emitting structure.
In comparison with a vertical-structure light emitting device in which two electrodes are respectively disposed on upper and lower surfaces of a light emitting structure, such a planar-structure nitride light emitting device cannot uniformly spread current in the entire light emission area thereof, thus having a not-so-large effective area for light emission and low light emission efficiency per light emission area. Such a problem of non-uniform current spreading is more serious in a larger light emitting device for illumination purpose which requires high output.
As a solution for the problem of current spreading, the electrode structure is extended in the entire area of the device as in the case of a nitride semiconductor light emitting device shown in FIGS. 1(a) and 1(b).
Referring to FIG. 1(a) together with FIG. 1(b), there is shown a nitride semiconductor light emitting device 10 in which a first conductivity type nitride semiconductor layer 12, an active layer 14 and a second conductivity type nitride semiconductor layer 15 are deposited in their order on a substrate 11. A transparent electrode layer 16 may additionally be formed on the second conductivity type nitride semiconductor layer 15 for an ohmic contact.
A portion of an upper surface of the nitride semiconductor light emitting device, where a first electrode is to be formed and connected to the first conducting nitride semiconductor device, is mesa-etched to expose a corresponding portion of an upper surface of the first conductivity type nitride semiconductor layer. Thus, the first and second electrodes 18 and 19 are formed respectively on the exposed portion of the first conductivity type nitride semiconductor layer and the exposed portion of the second conductivity type nitride semiconductor layer (More specifically, the transparent electrode layer 16).
The first electrode 18 is composed of a first bonding pad 18a and first extension electrodes 18b extended from the first bonding pad 18a, whereas the second electrode 19 is composed of a second bonding pad 19a and a second extension electrode 19b extended from the second bonding pad 19a. As shown in FIG. 1a, the first and second bonding pads 18a and 19a are disposed at opposing ends of the device 10, respectively. The first extension electrodes 18b are extended along opposing longitudinal sides, toward the second bonding pad 19a. The second extension electrode 19b is extended along a center of the device 10 such that the first extension electrodes 18b are positioned at the opposing sides about the second extension electrode 19b at a predetermined interval. Thereby, the first and second electrodes 18 and 19 can be disposed in a relatively regulated interval using the first and second extension electrodes 18b and 19b. 
However, as the first and second bonding pads 18a and 19a are formed in relatively large areas for wire bonding, it is almost impossible to form the first and second electrodes 18 and 19 at a perfectly regulated interval. For example, as shown in FIG. 1, even if the first and second extension electrodes 18b and 19b are disposed at a predetermined interval d1 from each other, since the second bonding pad 19a has relatively a large area, the interval d2 between the second bonding pad 19a and an adjacent portion of the first extension electrode 18b is narrower. In general, current flow tends to be concentrated in a region of small resistance, and thus even if the interval is regulated using the extension electrodes, the current is concentrated in a region adjacent to the large bonding pad, thus hindering uniform current spreading.
Such a problem is more serious in a large light emitting device such as an LED for illumination purpose. And as stated above, it is hard to expect increased light emission efficiency with increased area of the device.